This invention relates to optical amplifiers. More particularly, the invention relates to controlling gain characteristics of an optical amplifier in the extended or longer wavelength region of the gain spectrum by controlling the amplifier""s operating conditions.
The ability to affect spectral gain shape in a predictable and significant manner would be of great use in the design and control of optical amplifiers and, particularly, erbium-doped fiber amplifiers. Expanding the spectral bandwidth in which optical telecommunications signals can be transmitted is also highly desirable. In an erbium-doped amplifier the conventional gain bandwidth, which is commonly referred to as the C-band, extends from approximately 1520 nm to approximately 1560 nm thus providing approximately 40 nm of bandwidth for optical transmission signals but the erbium C-band has inherent limitations. They include a limited bandwidth and non-uniform gain over the C-band which characterizes typical amplifier systems. A discussion of these issues can be found in U.S. Pat. No. 6,320,693 by Cereo et al. entitled xe2x80x9cThermal Tuning of Optical Amplifiers and Use of Same in Wavelength Division Multiplex Systems,xe2x80x9d filed on Jun. 29, 1999 (hereinafter referred to as the xe2x80x9cCereo applicationxe2x80x9d), which is incorporated by reference herein as if set forth in its entirely. One approach to increase the useable transmission signal bandwidth of erbium-doped amplifiers is to take advantage of the spectral region from approximately 1565 nm to approximately 1620 nm which is commonly referred to as the erbium extended band or L-band (hereinafter referred to as the L-band).
A long-standing belief with respect to erbium-doped optical amplifiers was that the gain of the amplifier is essentially homogeneous in character and could be described by the homogeneous model discussed in, for example, C. R. Giles et al. xe2x80x9cModeling Erbium-Doped Fiber Amplifiers,xe2x80x9d Journal of Lightwave Technology, vol. 9, pp. 271-283 (1991), and C. R. Giles et al. xe2x80x9cOptical Amplifiers Transform Long Distance Lightwave Telecommunication,xe2x80x9d Proc. IEEE, vol. 84, pp. 870-883 (1996). The essence of this assumption is that the gain of an amplifier is determined by the average inversion of the active species (for example, the erbium ions in an erbium-doped fiber amplifier) irrespective of the particular signal wavelengths, signal powers, pump wavelength, and pump power which produced that average inversion. In other words, the assumption of homogeneous broadening means that if the gain at any one wavelength is by some means stabilized to a particular value, then a gain at the other wavelengths is similarly stabilized (the stabilized value of the gain being different at different wavelengths). By means of this assumption, a gain spectrum for an amplifier is calculated for a given average inversion.
While the homogeneous gain model may sufficiently describes the wavelength region from about 1540 nm to about 1565 nm, commonly referred to as the xe2x80x9cred band,xe2x80x9d it has been found that this model does not work well in the xe2x80x9cblue bandxe2x80x9d which extends from approximately 1525 nm to about 1545 nm. This is discussed in more detail in U.S. Pat. No. 6,144,486, entitled xe2x80x9cPump Wavelength Tuning of Optical Amplifiers and Use of Same in Wavelength Division Multiplex Systems,xe2x80x9d by Bennett et al., which was filed on Jan. 30, 1998 (hereinafter referred to as the xe2x80x9cBennett applicationxe2x80x9d), which is hereby incorporated by reference as if set forth in its entirety herein. The blue band exhibits substantial inhomogeneous behavior. For example, when at least one signal wavelength is in this band the gain spectrum can no longer be described by a single average inversion that applies to all active species. Inhomogeneous gain saturation is manifested in the C-band by the well-known phenomenon of spectral hole burning wherein a narrow bandwidth dip occurs in the gain spectrum at the saturating wavelength. Moreover, a temperature-induced gain saturation phenomenon known as xe2x80x9cthermal wigglexe2x80x9d has also been observed in the erbium C-band. This is described in the above-referenced Cereo application. Manifestations of spectral hole-burning and thermal wiggle in the L-band operating environment are quite distinct from the manifestations in the C-band. Accordingly, there is a need for a method of controlling the L-band gain spectrum due to the inhomogeneous broadening observed therein.
Gain tilt in WDM transmission systems is also a concern. In general terms, the gain spectrum G(xcex) of an optical amplifier is a function of a variety of variables, including input powers at the signal wavelengths xcex1 through xcexn, (signal wavelengths may themselves vary from application to application and thereby affect the gain spectrum); pump power and pump wavelength, the average inversion of the fiber (itself a function of the input powers and the pump power, as well as the length of the fiber), the temperature of the amplifying medium, and various other variables. Gain tilt is the term used in the art to describe the fact that under different operating conditions, an optical amplifier will amplify different channels to different relative extents. Although various changes in operating conditions can be considered, a particularly important change is that which occurs when the level of signal power at one, or more of the signal wavelengths changes. For example, the signal power at all of the signal wavelengths will change as the distance between amplifiers along a transmission line changes, e.g., the power will go down as the distance increases.
For the simplest case of a two-channel system, the gain tilt (GT) between operating condition O1 and operating condition O2 can be written:
GTO1xe2x86x92O2(xcex1, xcex2)=xcex94GO1xe2x86x92O2(xcex1)/xcex94GO1xe2x86x92O2(xcex2)
where xcex94GO1xe2x86x92O2(xcex1) and xcex94GO1xe2x86x92O2(xcex2) are, respectively, the changes in gain at xcex1 and xcex2 in going from operating condition O1 to operating condition O2 and the units of gain tilt are dB/dB.
A gain tilt of 1.0 means that the change in gains at xcex1 and xcex2 are the same so that if the gain spectrum of the amplifier was substantially flat (i.e., substantially free of ripple) for signals at xcex1 and xcex2 for operating condition O1, it will also be substantially flat for operating condition O2.
In practice, however, the gain tilt is not equal to 1.0. Instead, a change in operating conditions results in an increase in gain for some wavelengths (e.g., shorter wavelengths) relative to other wavelengths (e.g., longer wavelengths). That is, a plot of G(xcex) versus xcex appears to have undergone a rotation (a xe2x80x9ctiltingxe2x80x9d) in either a clockwise direction (if the gain at shorter wavelengths is increased relative to the gain at longer wavelengths) or counterclockwise direction (if the gain at longer wavelengths is increased relative to the gain at shorter wavelengths) as a result of the change in operating condition. Hence the name xe2x80x9cgain tilt.xe2x80x9d In addition to a rotation, the plot of G(xcex) versus xcex can undergo a net upward or downward shift along the vertical gain axis as a result of the change in operating conditions. Also, localized changes at particular wavelengths can occur affecting ripple.
Rotation of the gain spectrum with a change in operating conditions is a problem in WDM systems since any passive system that is designed to equalize the power output of the channels for one specific set of operating conditions is likely to fail to provide equalization when those conditions are changed.
Accordingly, there is a need to control this gain tilt phenomenon in the L-band.
An embodiment of the invention provides a method of operating an erbium doped optical amplifier for amplifying a signal in a spectral region above about 1565 nm. This method includes the step of controlling the shape of the gain spectrum of the optical amplifier by nonhomogeneously saturating the gain of the amplifier. It may include the step of positioning a gain saturating signal at a selected wavelength in the spectral region, or the step of positioning a single gain saturating signal at a single selected wavelength in the spectral region. In another aspect, the invention may further include the step of changing the nonhomogeneous gain saturation (NGS) at a particular saturating wavelength by pumping the amplifier in a pump band of approximately 980 nm or approximately 1480 nm.
In another aspect of the invention, the pumping in the 980 nm band may result in decreased NGS for saturating signals in a shorter wavelength region and increased NSG for saturating signals in a longer wavelength region relative to pumping in the 1480 nm band.
Another aspect of the invention is directed to a method of operating a long-band erbium doped optical amplifier which includes a gain medium for amplifying a signal in an L-band spectral region from about 1565 nm to about 1620 nm of an associated L-band gain spectrum of the amplifier. This method may include the step of controlling the shape of the gain spectrum of the optical amplifier by controlling the temperature of at least a portion of the gain medium in a manner which saturates the gain of the amplifier. Controlling the temperature may include the step of varying the temperature of the gain medium in a range from about xe2x88x9210 to about 80 degrees Celsius.
The invention may apply to any optical amplifier having a rare earth-doped, non-homogeneously broadened gain medium in which amplification is produced by stimulated emission, such as an erbium doped fiber amplifier.
The optical amplifier to which the method according to the invention applies may preferably an optical fiber amplifier. The amplifier may alternatively be a planar amplifier or other type amplifier most suitable to available manufacturing and assembly techniques. The method according to the invention may also apply to single and/or multi-stage optical amplifiers, hybrid amplifiers, and gain media compositions that may take the form of glass and/or glass ceramics and compositions including silicates, borates, and the like, and ZBLAN and variations thereof.